Article (mine) is covered by customs duty ultimately, alas (5,3,4)
I believe the answer is:
mores the pity
'article mine' is the definition.
The definition suggests a singular noun which matches the answer.
'covered by customs duty ultimately alas' is the wordplay.
'covered by' indicates putting letters inside.
'customs' becomes 'mores' (as in social mores).
'ultimately' indicates one should take the final letters.
'alas' becomes 'thepit' (I am not sure about this - if you are sure you should believe this answer much more).
The final letter of 'duty' is 'y'.
'mores'+'y'='moresy'
'moresy' placed around 'thepit' is 'MORES THE PITY'.
'is' acts as a link.
Can you help me to learn more?
(Another definition for mores the pity that I've seen is " And that's regrettable".)